Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element).
An inorganic electroluminescent element has been utilized as a flat light source, however, requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescent element is an element provided with a constitution comprising an emission layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emission layer to be recombined, resulting emission utilizing light release (fluorescence phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
In an organic electroluminescence in view of the future practical application, desired has been development of an organic EL element which efficiently emits at a high luminance with a low electric consumption. Examples of such technologies are a slight amount of a fluorescent substance doped in a stilbene derivative, distyrylarylene derivative or a tristyrylarylene derivative, to achieve improved emission luminance and a prolonged lifetime of an element (for example, refer to Patent Document No. 1). Further, there are known such as an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with a slight amount of a fluorescent substance (for example, refer to Patent Document No. 2). and an element having an organic emission layer comprising a 8-hydroxyquinoline aluminum complex as a host compound which is doped with quinacridone type dye (for example, refer to Patent Document No. 3).
Regarding to the technologies disclosed in the above-described Patent Documents, when emission from an excited singlet is utilized, since a generation ratio of a singlet exciton to a triplet exciton is ⅓, that is, a generation probability of an emitting exciton species is 25% and a light taking out efficiency is approximately 20%, the limit of a quantum efficiency (ηext) of taking out is said to be 5%.
However, since an organic EL element which utilizes phosphorescence from an excited triplet has been reported from Princeton University (for example, refer to Non Patent Document 1), researches on materials exhibiting phosphorescence at room temperature have come to be active (for example, refer to Non Patent Document No. 2 and Patent Document No. 4).
Since the upper limit of internal quantum efficiency becomes 100% by utilization of an excited triplet, which is principally 4 times of the case of an excited singlet, it may be possible to achieve almost the same ability as a cooled cathode ray tube to attract attention also for an illumination application.
For example, many compounds mainly belonging to heavy metal complexes such as iridium complexes have been synthesized and studied (for example, refer to Non Patent Document No. 3). Further, utilization of tris(2-phenylpyridine)iridium as a dopant has been studied (for example, refer to Non Patent Document No. 2).
In addition to these, there have been studied to use L2Ir(acac) such as (ppy)2Ir(acac) as a dopant (for example, refer to Non Patent Document No. 4). Also there have been studied to use compounds such as tris(2-(p-tolyl)pyridine) iridium (Ir(ptpy)3) and tris(benzo[h]quinoline)iridium (Ir(bzq)3), Ir(bzq)2 CIP(Bu)3 (for example, refer to Non Patent Document No. 5).
Further, to obtain high emission efficiency, a hole transporting compound is known to use as a host of a phosphorescent compound (for example, refer to Non Patent Document No. 6). Further, various types of electron transporting materials have been used as a host of a phosphorescent compound doped with a new iridium complex for example, refer to Non Patent Document No. 4). In addition, a high emission efficiency has been achieved by introduction of a hole block layer (for example, refer to Non Patent Document No. 5).
Over recent years, there has been proposed a method of enhancing the conductivity of an organic layer by increasing the carrier concentration in a positive hole transport layer and in an electron transport layer which are in thermal equilibrium, wherein acceptors are doped in the positive hole transport layer and donors are doped in the electron transport layer (refer, for example, to Patent Documents 5 and 6).
Further, there have been proposed an organic electroluminescent element incorporating a donor-doped electron transport layer and a positive hole inhibition layer which efficiently traps positive holes, as well as an organic electroluminescent element incorporating an acceptor-doped positive hole transport layer and an electron inhibition layer which efficiently traps electrons, resulting in an organic electroluminescent element featuring a constitution which excels in electro-optic characteristics exhibiting further enhanced emission efficiency (refer, for example, to Patent Document 7).
An organic EL element is further characterized by exhibiting extensive color variations, and is also characterized by emitting light of various colors by color mixing in combinations of plural emission colors.
Of the emission colors, in particular, white color emission is highly demanded, which can also be utilized as a backlight for a display device. Further, white color emission is separable into blue, green, and red pixels using appropriate color filters.
As such a method for emitting white light, the following two methods are applicable.
(a) Plural emission compounds are doped in one emission layer.
(b) Plural emission colors from plural emission layers are combined.
For example, in cases in which white color is formed using the three colors of blue (B), green (G), and red (R), with regard to method 1, a four-way deposition for B, G, and R light emission materials as well as for a host compound is required, provided that a vacuum deposition method is employed as an element production method. However, there is much difficulty in controlling the four-way deposition.
Further, although there is a method of coating B, G, and R light emission materials as well as a host compound after dissolving the same in a solvent or dispersing the same, there has, so far, been the continuing problem that a coating type organic EL element is inferior in layer durability to a deposition type.
In contrast, a method of combining plural emission layers, described in method 2, has been proposed. In cases when employing the deposition type organic EL element, method 2 is more readily employed than method 1.
With regard to such an organic EL element emitting white light, an attempt to obtain white light emission by color mixing using both of the following emission layers has been proposed, wherein the emission layers are formed via lamination of appropriate layers, which are a blue light emission layer for short wavelength emission and a yellow light emission layer for long wavelength emission (refer, for example, to Patent Document 8).
However, in an organic EL element formed via lamination of these two emission layers which each emit different colors (namely each featuring different peak wavelengths), the emission center is likely to shift due to variations in the layer quality of the two emission layers or in transportability of holes (positive holes) and electrons, according to variations in the driving duration of the element, that is, variations in the emission duration or in the applied voltage, whereby chromaticity variation tends to Occur.
Specifically, when white color is formed by color mixing using the two emission layers, white color is sensitive to chromaticity variation, compared with the other colors, resulting in the above problem.
In an organic EL element featuring mixed color emission from plural emission layers each of which has a different peak wavelength, those formed by alternately laminating at least three layers, which each emit light at different peak wavelengths, have been disclosed as a method of inhibiting, as much as possible, the chromaticity variation due to the driving duration variation or the voltage variation (refer, for example, to Patent Document 9).
Further, in a laminated structure of at least two layers, a method of designing the layer thickness of the emission layer and the ratio of an organic host material and a fluorescent material, as parameters, has been disclosed (refer, for example, to Patent Document 10).
Alternate types of lamination thereof make it possible to realize a preventive effect of chromaticity shift even when the carrier injection balance slightly varies. However, due to poor emission efficiency and the occurrence of interlayer energy transfer, uneven distribution of whiteness is noted, resulting in inferior white light emission.    Patent Document 1: Japanese Patent Publication No. 3093796 specification    Patent Document 2: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 63-264692    Patent Document 3: JP-A No. 3-255190    Patent Document 4: U.S. Pat. No. 6,097,147 specification    Patent Document 5: JP-A No. 4-297076    Patent Document 6: JP-A No. 2001-244079    Patent Document 7: JP-A No. 2000-196140    Patent Document 8: JP-A No. 7-142169    Patent Document 9: JP-A No. 2003-187977    Patent Document 10: JP-A No. 2004-63349    Non-Patent Document 1: M. A. Baldo et al., Nature, Vol. 395, pages 151-154 (1998)    Non-Patent Document 2: M. A. Baldo et al., Nature, Vol. 403, No. 17, pages 750-753 (2000)    Non-Patent Document 3: S. Lamansky et al., J. Am. Chem. Soc., Vol. 123, page 4304 (2001)    Non-Patent Document 4: M. E. Tompson et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)    Non-Patent Document 5: Moon-Jae Youn. Og, Tetsuo Tsutsui et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)    Non-Patent Document 6: Ikai et al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL' 00, Hamamatsu)